Internet-based protests against a pair of controversial anti-piracy bills gained momentum Wednesday, as several lawmakers dropped their support in the face of widespread opposition from the tech industry. Until recently, the Stop Online Piracy Act (SOPA) and the Protect IP Act (PIPA), both aimed at cracking down on web piracy, seemed destined for certain passage. The bills enjoyed wide support in Congress, as well as strong backing from Hollywood, the recording industry, and major media companies. But a surprisingly strong web-based opposition campaign, spurred in part by the Internet industry, has put the bills’ supporters on the defensive.

Neither SOPA or PIPA is dead, but the intense push-back demonstrates the increasing power of web-based activists in debates over public policy and commerce. On Wednesday, as thousands of websites went black to protest the measures, Sen. Marco Rubio, a Florida Republican, withdrew his support for PIPA, which he had co-sponsored. Shortly thereafter, Sen. John Cornyn, a Texas Republican, also reversed course and urged his colleagues to take more time to examine the proposed legislation. Several other lawmakers also voiced their opposition, Politicoreported.

For years, large entertainment and content companies have been urging Congress to crack down on web piracy, which they say costs movie studios, record labels and other content producers billions of dollars annually. SOPA (the House version) and PIPA (the Senate version) would shift the burden for policing web piracy onto Internet companies and away from rights-holders, as is the case under an existing law called the Digital Millennium Copyright Act (DMCA). Under the DMCA, internet companies can’t be held liable if they promptly remove infringing content upon notification. Many industry experts credit this so-called “safe harbor” provision with helping to foster the rapid innovation and explosive growth of the Internet over the last 15 years.

Although the two bills target foreign sites, domestic internet companies like Google (and its YouTube video site), Facebook and Twitter could be held responsible if they unknowingly link to infringing content, according to critics. Earlier, more extreme versions of the bills would have required Internet Service Providers and search engines to block access to sites that run afoul of copyright laws, essentially creating an Internet blacklist, and a powerful tool for web censorship.

SOPA and PIPA have the support of major content companies (including Time Warner, owner of TIME), but the bills have provoked howls of protest from powerful Internet firms and industry figures. Sergey Brin, cofounder of Google, said last month that he was “shocked that our lawmakers would contemplate such measures that would put us on a par with the most oppressive nations in the world.” On Wednesday, Google draped a black box on its homepage to protest the bills. Several thousand websites went dark, including Wikipedia and the online community Reddit, which has been a hotbed of anti-SOPA organizing. “Fighting online piracy is important,” Google said in a statement. “The most effective way to shut down pirate websites is through targeted legislation that cuts off their funding. There’s no need to make American social networks, blogs and search engines censor the Internet or undermine the existing laws that have enabled the Web to thrive, creating millions of U.S. jobs.”

Anti-SOPA forces have used Twitter, Facebook and other web tools to build grassroots opposition to the bills, and it appears to be working. Late last week, Rep. Lamar Smith, a Texas Republican who chairs the House Judiciary Committee, said he would cut SOPA’s web-blocking provisions, and Sen. Pat Leahy, a Vermont Democrat who leads the Senate Judiciary Committee, said he would remove similar language from PIPA. Just days later, Rep. Darrell Issa, a California Republican, said he had been assured by Majority Leader Eric Cantor, a Virginia Republican, that lawmakers will “continue to work to address outstanding concerns and work to build consensus prior to any anti-piracy legislation coming before the House for a vote.”

Then, over the weekend, the White House made clear that it opposes the more extreme measures contained in the bills. “While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cybersecurity risk or undermines the dynamic, innovative global Internet,” three top White House officials, Aneesh Chopra, Victoria Espinel and Howard Schmidt, wrote in an official blog post.

Supporters of SOPA and PIPA appear to have been taken aback by the strength of the opposition, but it’s clear that they aim to keep pushing the bills through. Over the weekend, News Corp. Chairman Rupert Murdoch took to Twitter to blast President Obama and Google for their opposition to the bills. “So Obama has thrown in his lot with Silicon Valley paymasters who threaten all software creators with piracy, plain thievery,” Murdoch wrote. Later, he declared: “Piracy leader is Google who streams movies free, sells advts [advertisements] around them. No wonder pouring millions into lobbying.” (Google called the charge “nonsense.”) Rep. Smith, meanwhile, has announced that work on SOPA will resume in February, although it’s unclear what form the legislation will take.

For now, anti-SOPA activists can take some satisfaction in their new-found clout, while the bill’s proponents re-group. Although the war is far from over, the bill’s opponents have put Hollywood, the recording industry, the major media companies — not to mention their supporters in Congress — on notice that they won’t roll over without a fight.